Method and apparatus for pre-loading a piezoelectric transducer for downhole acoustic communication

ABSTRACT

A downhole acoustic transmitter has a pre-loaded piezoelectric transducer, an enclosure in which the piezoelectric transducer is housed, a preload spring that biases the transducer against a first end coupling of the enclosure, and an adjustable preload means mounted to the enclosure such that a selected compressive force is applied to the preload spring, which in turn urges the transducer against a face of the first end coupling such that a mechanical preload is applied to the transducer. The position of the adjustable preload means and the spring compliance are selected so that the level of mechanical preload applied to the transducer compensates for an expected amount of flexing of the acoustic telemetry transmitter due to varying tension and compression applied to the transmitter, thereby maintaining an effective preload on the transducer.

FIELD

This disclosure relates generally to a downhole acoustic transmitter having a pre-loaded piezoelectric transducer and a method for pre-loading a piezoelectric transducer for use in downhole communication such as downhole acoustic telemetry.

BACKGROUND

The evolution of modern oil and gas wells has led to increases in both the depth of the wells and the complexity of the procedures and equipment needed for drilling and completions operations. Additionally, there is an ongoing need for improved safety and efficiency in the drilling and completions process. The combination of these factors has created a need for improved visibility of the downhole conditions along the length of the drill string and at the bottom hole assembly (BHA) during drilling and completions operations. Downhole sensor measurements such as downhole bore and annular pressure, drill string torque and tension, and temperature can be transferred from a downhole location to the surface through one of several known telemetry methods.

One type of downhole communication method is wired drill pipe telemetry, which offers very high bandwidths, but tends to be expensive to deploy and prone to failure. Another known downhole communication method is mud pulse telemetry which encodes sensor data into pressure waves that are induced in the drilling fluid flowing in the drill string. Drawbacks to mud pulse telemetry include an inability to transmit when drilling fluid is not flowing, and relatively low data rate transmissions which decrease as the depth of the well increases. A third type of downhole communication is electromagnetic (EM) telemetry, which transmits digitally modulated electromagnetic waves through the formations surrounding the drill string to a surface receiver. EM telemetry does not require the flow drilling fluid and can provide a higher data transmission rate than mud pulse telemetry, but can be sensitive to the nature of the formations surrounding the well and may not be well suited for deeper wells.

A fourth type of downhole communication is acoustic telemetry, which has proven to be well suited for the modern drilling environment. Acoustic telemetry is capable of transmitting hundreds of bits per second, and since it uses the body of the drill pipe as its transmission medium it is insensitive to the surrounding formation or casing, and does not require any fluid flow to enable the transmission of data.

There are currently three different implementations of acoustic telemetry systems in downhole tools that use acoustic telemetry: probe-based, clamp-on, and collar-based. These systems typically comprise components including sensors, electronics, batteries and an acoustic transmitter. The probe-based implementation is mounted at least partially within the bore of the drill pipe. The clamp-on implementation is mounted on the external wall of the drill pipe. The collar-based implementation places the components within an annular space in the downhole tool.

In a typical drilling or completions environment, a number of acoustic transmitters can be spaced along the length of the drill string. The most common type of acoustic transducer used within downhole tools comprises a cylindrical piezoelectric stack mounted in a collar-based implementation. Such a stack comprises a number of thin piezoceramic discs layered with thin electrodes between each disc which are connected electrically in parallel. As is known in the art, such as disclosed in U.S. Pat. No. 6,791,470, the entirety of which is incorporated by reference herein, an advantage of the piezoelectric stack when compared to other acoustic transducer types is that the acoustic impedance of the stacked ring structure can be closely matched to the acoustic impedance of the tool's structure thereby optimizing the transfer of acoustic energy from the stack into the tool body, and subsequently into the drill string. Any acoustic impedance mismatch between the stack and the tool surrounding structure results in a reduction in the acoustic output power of the tool.

The piezoelectric stack structure offers a large displacement force combined with a high energy conversion efficiency and high compressive strength, but offers little resistance to tension, even that incurred when voltage is applied. Due to its low tensile strength, it is common practice to place a piezoelectric stack under a mechanical compressive preload along the stack's axis of operation in order to maintain stack integrity while being actuated. The magnitude of the preload can compensate for dynamic forces, but also affects the mechanical energy output from the stack. If there is no compressive preload or if the compressive preload exceeds the blocking force of the piezoelectric material, then there is no mechanical energy output from the stack. An optimum preload level that will maximize the output mechanical energy from the stack occurs when the stiffness of the preloaded stack is equal to the stiffness of the mechanical load.

Referring to FIG. 1, a prior art collar-based piezoelectric stack-type acoustic transmitter 301 comprises first and second thermal expansion compensation rings 302 a and 302 b, a retaining ring 303, end coupling 304, a steel outer housing 305, a mandrel 306, a pin 307, and a piezoelectric stack 308. The first and second thermal expansion rings 302 a and 302 b are designed to compensate for the difference between the thermal expansion of the steel housing 305 and the piezoelectric stack 308. The mandrel 306 is threaded into the end coupling 304, and the first thermal expansion compensation ring 302 a is slid down the mandrel 306 to an inner face 309 of the end coupling 304. The piezoelectric stack 308 is slid down the mandrel 306 to rest against the first thermal compensation ring 302 a. The second thermal compensation ring 302 b is slid down the mandrel 306 to rest against the end of the piezoelectric stack 308, and the retaining ring 303 is placed on the mandrel 306 against the second thermal compensation ring 302 b. The outer housing 305 is placed over the mandrel 306, first and second thermal compensation rings 302 a, 302 b and the retaining ring 303 and threaded onto the end coupling 304. The pin 307 is threaded into the housing 305 until the thread is shouldered, and the inner face of the pin 310 is forced against the retaining ring 303 which in turn forces the thermal compensation rings 302 a, 302 b and the piezoelectric stack 308 against the immoveable inner face 309 of the end coupling 304, thereby creating a compressive preload force on the piezoelectric stack 308. The amount of compressive force on the piezoelectric stack can be controlled by varying the length of the retaining ring 303.

The prior art acoustic transmitter 301 will maintain a positive compressive preload on the piezoelectric stack 308 over a limited range of tension/compression on the downhole tool. However, in deeper wells such as those drilled offshore, the tension/compression applied to the downhole tool by external forces can result in the tool flexing enough to either reduce the preload to zero, or to compress the piezoelectric stack beyond its compressive limits. Thus there is a need for a method of applying a compressive preload to the piezoelectric stack in a downhole acoustic transmitter that will maintain an effective preload over the entire range of tension and compression applied to the downhole tool by the drill string while operating in a downhole environment.

SUMMARY

According to one aspect, there is provided a downhole acoustic transmitter for use in downhole communication, comprising an enclosure, a piezoelectric transducer, a preload spring and an adjustable preload means. The enclosure comprises a first end coupling, a second end coupling, a tubular outer housing having a first end coupled to the first end coupling and a second end coupled to the second end coupling, and an inner mandrel inside the outer housing and extending between the first and second end couplings such that an annular space is defined between the mandrel and the outer housing. The piezoelectric transducer is in the annular space, and has a first end contacting an inner face of the first end coupling in an axial direction. The preload spring is in the annular space and has a first end contacting a second end of the piezoelectric transducer in the axial direction. The adjustable preload means contacts the enclosure and a second end of the preload spring such that a compressive force in the axial direction is applied to the preload spring, which in turn compresses the piezoelectric transducer against the inner face of the first coupling.

The adjustable preload means can comprise one or more spacers contacting an inner face of the second end coupling, or be a retaining ring attached to an inner surface of the outer housing, or be a threaded nut attached to the mandrel. The piezoelectric transducer can comprise an annular stack of annular piezoceramic discs with annular electrodes between each disc, wherein the annular stack is slidable over the mandrel. The preload spring can be a metal tube slidable over the mandrel, or can be one or more metal rods or tubes each extending in the axial direction in the annular space.

The downhole acoustic transmitter can further comprise an acoustic tuning element in the annular space and attached to the second end of the piezoelectric transducer. The acoustic tuning element has a selected acoustic impedance that when combined with the acoustic impedance of the preload spring, equals the acoustic impedance of the inner face of the first end coupling. The acoustic tuning element can comprise a metal cylinder having a first end attached to the second end of the piezoelectric transducer and a free second end. One or more of a mass density, mass distribution, length and cross sectional area of the acoustic tuning element can be selected to provide the selected acoustic impedance.

According to another aspect, there is provided a downhole acoustic telemetry node which comprises one or more sensors for measuring a local borehole environment and one or more mechanical conditions of a drill string (e.g. pressure, temperature, tension, compression and torque), a processor and memory communicative with the one or more sensors for storing measurements taken by the one or more sensors, and the downhole acoustic transmitter, which is communicative with the processor and memory and is operable to transmit the measurements.

According to another aspect, there is provided a method for acoustic transmission from a downhole location, comprising: (a) applying a compressive preload in an axial direction against a preload spring, which in turn compresses a piezoelectric transducer against an inner face of a first end coupling of an enclosure of a downhole acoustic transmitter, wherein the compressive preload is selected to place the piezoelectric transducer in compression over a range of expected operating conditions of the downhole acoustic transmitter; and (b) applying a voltage to the piezoelectric transducer to generate an acoustic transmission. The method can further comprise tuning the acoustic impedance of the piezoelectric transducer by contacting an end of the piezoelectric transducer with an acoustic tuning element having a selected acoustic impedance such that when combined with an acoustic impedance of the preload spring, equals the acoustic impedance of the inner face of the first end coupling, wherein the end of the piezoelectric transducer contacting the acoustic tuning element also contacts the preload spring.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic side sectioned view of a downhole acoustic transmitter used in a downhole acoustic communication system (PRIOR ART).

FIG. 2 is a schematic representation of a drill string comprising a downhole acoustic communication system according to embodiments of the invention.

FIG. 3 is a frequency response graph of a modulated acoustic signal transmitted by the downhole acoustic communication system of FIG. 2.

FIG. 4 is a schematic side sectioned view of a downhole acoustic transmitter comprising a preload spring compressed by adjustable spacers according to one embodiment of the invention.

FIG. 5 is a schematic side sectioned view of a downhole acoustic transmitter comprising a preload spring compressed by an adjustable retaining ring according to another embodiment of the invention.

FIG. 6(a) is a schematic side sectioned view of a downhole acoustic transmitter comprising a preload spring compressed by an adjustable retaining ring and an acoustic tuning element according to another embodiment of the invention, and FIG. 6(b) is a detail view of an interface of the acoustic tuning element and a transducer stack of the downhole acoustic transmitter.

FIG. 7 is a schematic side sectioned view of a downhole acoustic transmitter comprising a preload spring compressed by an adjustable threaded nut according to another embodiment of the invention.

FIG. 8 is a schematic side sectioned view of a downhole acoustic transmitter comprising a preload spring compressed by an adjustable threaded nut and an acoustic tuning element according to another embodiment of the invention.

FIG. 9(a) is a graph showing a first resonance peak in an example steel cylinder having a first constrained end and a second free end, and FIG. 9(b) is a graph showing the magnitude of the cylinder's acoustic impedance across a third acoustic passband of the drill string as shown in FIG. 3.

FIG. 10 is a graph of the acoustic impedance of a piezoelectric stack and the acoustic impedance of an acoustic tuning element of an example downhole acoustic transmitter.

FIG. 11 is an acoustic amplitude vs. frequency graph illustrating the acoustic output of a prior art acoustic transmitter having a fixed preload, the acoustic transmitter with a preload spring and an adjustable preload means as shown in FIG. 5, and the acoustic transmitter with a preload spring, adjustable preload means and an acoustic tuning element as shown in FIGS. 6(a) and (b).

DETAILED DESCRIPTION

Directional terms such as “top”, “bottom”, “upwards”, “downwards”, “vertically”, and “laterally” are used in the following description for the purpose of providing relative reference only, and are not intended to suggest any limitations on how any article is to be positioned during use, or to be mounted in an assembly or relative to an environment.

Additionally, the term “couple” and variants of it such as “coupled”, “couples”, and “coupling” as used in this description is intended to include indirect and direct connections unless otherwise indicated. For example, if a first device is coupled to a second device, that coupling may be through a direct connection or through an indirect connection via other devices and connections. Similarly, if the first device is communicatively coupled to the second device, communication may be through a direct connection or through an indirect connection via other devices and connections.

Furthermore, the singular forms “a”, “an”, and “the” as used in this description are intended to include the plural forms as well, unless the context clearly indicates otherwise.

The embodiments described herein relate generally to a downhole acoustic transmitter having a pre-loaded piezoelectric transducer and a method for pre-loading a piezoelectric transducer for use in downhole acoustic communication such as downhole telemetry. The transmitter comprises an enclosure in which the piezoelectric transducer is housed, a preload spring that biases the transducer against a first end coupling of the enclosure, and an adjustable preload means mounted to the enclosure such that a selected compressive force is applied to the preload spring, which in turn urges the transducer against a face of the first end coupling such that a mechanical preload is applied to the transducer. The position of the adjustable preload means and the spring compliance are selected so that the level of mechanical preload applied to the transducer compensates for an expected amount of flexing of the acoustic telemetry transmitter due to varying tension and compression applied to the transmitter, thereby maintaining an effective preload on the transducer.

In some embodiments, the downhole acoustic transmitter further comprises an acoustic tuning element positioned to contact the piezoelectric transducer at the same end as the preload spring. The acoustic tuning element is tuned such that the acoustic impedance seen by the piezoelectric transducer at that end, comprising the combination of the acoustic impedance of the tuning element and the acoustic impedance of the preload spring at that end, is equal to the acoustic impedance offered to the transducer at the other end by the face of the first end coupling, thereby maintaining the output power of the transducer while compensating for any variations in the mechanical preload applied by the preload spring.

Referring now to FIG. 2, one or more of the acoustic telemetry transmitters can be installed in a drill string. Drill string tubing 103 is suspended in a borehole 108 from a drilling rig 102. The tubing 103 can extend for thousands of feet, and in a typical deployment an acoustic transmitter is part of a telemetry tool 105 in a bottom hole assembly (BHA) 104. Additional acoustic transmitters can be included in repeaters 106 along the length of the tubing 103, with the number of repeaters 106 and the spacing between them determined by the along-string measurements required, if any, at each of the additional locations, and the possible necessity to repeat the acoustic signal if the distance to the surface is too far to transmit successfully with a single acoustic transmitter. The acoustic signal is received at the surface by a receiver 107.

The acoustic transmitters in this embodiment have a collar-based configuration, with the components of the acoustic transmitter including the piezoelectric transducer, sensors, electronics and batteries being mounted in a wall of a tubular section of the repeater 106 or the telemetry tool 105. However, the acoustic transmitters can have a probe-based or clamp-on configuration according to other embodiments (not shown). As will be described in more detail below, each acoustic transmitter comprises a mandrel defining a through-bore which allows fluid to pass through repeater 106 or telemetry tool 105. Each acoustic transmitter is operable to transmit a modulated acoustic signal as an extensional wave through the drill string components. The connection of several lengths of tubing 103 of similar size and dimensions is well known to form an acoustic frequency response similar to a bandpass comb filter which comprises a number of passbands alternating with stopbands as shown in FIG. 3. The bandwidth of the modulated acoustic signal is limited by the bandwidth of the acoustic passband used for the transmission, although more than one passband can be used to transmit simultaneously which increases the total bandwidth available for the signal and hence the data rate. The telemetry signal travels to the surface, either directly or through the repeaters 106, where it is received and decoded by the receiver 107.

According to a first embodiment and referring to FIG. 4, the acoustic transmitter 401 used in the telemetry tool 105 and repeater 106 generally comprises an enclosure, a transducer 405 housed within the enclosure, a preload spring 407 contacting one end of the transducer 405, and one or more spacers 409 which provide an adjustable means for applying a selected compressive load (herein referred to as “preload”) on the transducer 405 via the preload spring 407. The enclosure comprises a first end coupling 402, a tubular outer housing 403, a cylindrical inner mandrel 404 and a second end coupling 410 (also referred to as a “pin”). The first end coupling 402 has a body with threads on the outer surface of the body (“external threads”), and a central bore extending through the body. A first end of the inner mandrel 404 is externally threaded and engages internal threads in the central bore of the first end coupling 402 along a central axis. Both ends of the outer housing 403 are internally threaded, with an internally threaded first end engaging the external threads of the first end coupling 402 and an internally threaded second end engaging external threads of the second end coupling 410. The second end coupling 410 has a body with a bore extending through the body, and which engages a second end of the inner mandrel 404 by a threaded connection. When assembled, the enclosure defines a through bore that extends through the central bores of the end couplings 402, 410 and the bore of the mandrel 404, such that drilling fluid can flow through the acoustic transmitter 401. The assembled enclosure also defines a fluid-tight annular space 408 for housing the transducer 405, preload spring 407, and spacers 409.

The transducer 405 comprises a stack of thin annular piezoceramic discs layered with thin annular electrodes between each disc which are connected electrically in parallel (the transducer is herein alternatively referred to as a “piezoelectric stack” 405). As a result, the stack's electrical behavior is primarily capacitive. Applying a high voltage charges the piezoelectric stack 405 and causes it to increase and decrease in length. It is this deflection that launches extensional waves into the drill pipe (not shown). Data can be carried by the extensional waves by modulating the voltage applied to the piezoelectric stack 405.

The piezoelectric stack 405 slides over the mandrel 404 and has a first end that contacts an inner face of the first end coupling 402. The preload spring 407 is shown in FIG. 4 as a coil spring that slides over the mandrel 404 with a first end that contacts a second end of the piezoelectric stack 405. However, the preload spring 407 can alternatively have different forms, including a metal cylinder (not shown) of selected length and spring constant that slides over the mandrel 404, or one or more metal rods or tubes (not shown) that extend axially in the annular space between the mandrel 404 and the outer housing 403.

One or more spacers 409 slide over the mandrel 404 to contact a second end of the preload spring 407. The pin 410 is threaded onto the internally threaded second end of the outer housing 403 such that an inner face of the pin 410 applies axial pressure against the spacer(s) 409, which in turn applies an axial compressive preload against the piezoelectric stack 405. Although only one spacer 409 is shown in FIG. 4, additional spacers 409 can be inserted depending on the desired preload to be applied to the piezoelectric stack 405; that is, each spacer 409 has a certain thickness, and the more spacers 409 inserted between the pin end and the preload spring end, the higher the compressive preload will be applied to the transducer 405. The properties of the preload spring 407 are selected to provide a degree of compliance in the preload applied against the transducer 405, i.e. to mitigate against the varying external tensile and compressive forces imposed on the acoustic transmitter 401 during drill string operation.

That is, the physical environment imposed on the acoustic transmitter 401 can be particularly challenging, with the telemetry tool 106 in particular being subjected to extreme ranges of pressure, temperature, and tension/compression, all of which vary as a function of the tool's placement in the drill string, depth, and the rig's operational state. The orientation of the borehole 108 containing the tubing 103 can be vertical with an inclination of 0 degrees, or may have one or more deviations in orientation along its length resulting in changes of inclination as high as 90 degrees. Due to the length of the tubing 103 and the deviations in its orientation, the tensile and compressive forces that the telemetry tool 106 are subjected to during rig operations can be very high. For example, the telemetry tool 106 may be subject to pressures up to 30 kpsi, tensions over 1,000,000 pounds, and temperatures up to 175° C. Of particular concern to the piezoelectric stack 405 is the flexing of the tool structure under various load conditions. These varying load conditions can affect the mechanical energy output by the piezoelectric stack 405 as the compressive load on the piezoelectric stack 405 varies. In the extreme, the piezoelectric stack 405 can be depolarized due to excessive compression caused by compression on the tool 106, or be damaged when the stack compression falls below safe operating levels during periods of high tension on the tool 106.

Because a selected compressive preload is applied to the piezoelectric stack 405 by the spacers 409 via the preload spring 407, the piezoelectric stack 405 can be subjected to relatively large variations in compressive load as the tool 106 is subjected to changes in the drill string tension and compression during the rig's operations. The amount of compressive preload applied to the piezoelectric stack 405 by the preload spring 407 and spacers 409 can be selected by selecting the spring constant of the preload spring 407 and selecting the number of spacers 409 between the preload spring 407 and the pin 410. An appropriate compressive preload maintains a positive compressive preload on the stack 405 over the entire range of tension and compression expected to be applied to the telemetry tool 105 by the drill string during a drilling operation. Determining the appropriate preload will be evident to one skilled in art based on certain properties of the drill string, borehole, reservoir, and drilling operation. Once the appropriate preload is determined, a spring 407 with a suitable spring constant and a suitable number of spacers 409 can be selected to provide the appropriate preload.

Referring to FIG. 5 and according to another embodiment of the acoustic transmitter 501, a retaining ring 509 is used instead of spacers 409 to apply a compressive preload to a transducer 505 via a suitable preload spring 507. Like the first embodiment, this alternative embodiment also comprises an enclosure having first and second end couplings 502, 510, and an outer housing 503 and a mandrel 504 that connect to the end couplings 502, 510 to form a fluid-tight annular space 508 in which the transducer 505, preload spring 507 and retaining ring 509 are housed. The retaining ring 509 is fixedly mounted to the inner surface of the outer housing 503 in a location that provides the desired compressive preload to the transducer 505.

Referring to FIGS. 6(a) and (b) and according to another embodiment, an acoustic transmitter 601 has the same elements as the acoustic transmitter 501 shown in FIG. 5, and is further provided with an acoustic tuning element 606 that serves to match the acoustic impedance of the piezoelectric stack 605 with the first end coupling 602, thereby maintaining optimal power output by the acoustic transmitter 601. Like the embodiment shown in FIG. 5, the acoustic transmitter 601 generally comprises an enclosure, a transducer 605 comprising the piezoelectric stack, a preload spring 607, and a retaining ring 609 for applying an axial compressive preload on the transducer 605 via the preload spring 607. The enclosure comprises a first end coupling 602 with an inner face 611, a tubular outer housing 603, a cylindrical inner mandrel 604 and a second end coupling 610 (“pin”). The acoustic tuning element 606 has a metal tubular body with a first end for contacting the piezoelectric stack 605 and an open second end 612.

The acoustic tuning element 606 is slid over the mandrel 604 such that the first end attaches to the piezoelectric stack 605 by a threaded connection, while leaving an annular space 608 between the outer surface of the mandrel 604 and the inner face of the acoustic tuning element 606. The preload spring 607 is slid over the mandrel 604 into the annular space 608 between the mandrel 604 and the acoustic tuning element 606 to contact the end of the piezoelectric stack 605. The outer housing 603 is slid over the assembly and threaded onto the external threads of the first end coupling 602, and the retaining ring 609 is slid over the mandrel 604 and comprises external threads which engage with internal threads of the outer housing 603 such that a compressive preload is applied to the piezoelectric stack 605 via the preload spring 607; consequently the piezoelectric stack 605 is compressed between the preload spring 607 and the inner face 611 of the first end coupling 602. The retaining ring 609 does not contact the second end 612 of the tuning element 606; therefore, the second end 612 of the tuning element remains “open”. The pin 610 is threaded into the outer housing 603 and mandrel 604 to close and seal the annular space 608 but does not contribute to the preload on the piezoelectric stack 605.

The acoustic tuning element 606 comprises a resonant structure that is tuned such that when it is attached to the end of the piezoelectric stack 605 its acoustic impedance reduces the piezoelectric stack 605 compliance at the frequencies being transmitted, and restores the acoustic match between the piezoelectric stack 605 and the first end coupling 603 without affecting the preload applied to the piezoelectric stack 605 by the preload spring 607.

For optimal acoustic output power, the piezoelectric stack 605 should be matched at either end with acoustic impedances equal to that of the piezoelectric stack 605; however the additional compliance of the preload spring 607 reduces the acoustic impedance seen by the piezoelectric stack 605 at the end at which the preload force is applied. The acoustic impedance of a segment of a cylinder of length l can be determined using the four-pole matrix solution to the wave equation. The four-pole solution can be written as:

$\begin{bmatrix} {F\left( {x + l} \right)} \\ {V\left( {x + l} \right)} \end{bmatrix} = {\begin{bmatrix} {\cos ({kl})} & {{iz}\; {\sin ({kl})}} \\ \frac{i\; {\sin ({kl})}}{z} & {\cos ({kl})} \end{bmatrix}\begin{bmatrix} {F(x)} \\ {V(x)} \end{bmatrix}}$

in which

${k = \left( \frac{2\pi \; f}{c} \right)},$

where c is the wave speed which is defined as

$c = \sqrt{\frac{E}{\rho}}$

where E is the Young's modulus of the cylinder material and ρ is the mass density of the material. The force at one end of the cylinder at x+l can be written as

F(x+l)=F(x)cos(kl)+izV(x)sin(kl)  Equation 1

in which z is the wave impedance of the cylinder which is defined as z=ρca, and a is the cross sectional area of the cylinder. In the case of a cylinder with an open end F(x+l)=0, resulting in an acoustic impedance at the opposing end of the cylinder of:

$\begin{matrix} {{Z_{a}(x)} = {\frac{F(x)}{V(x)} = {{- \frac{{iz}\; {\sin ({kl})}}{\cos ({kl})}} = {{- {iz}}\; {\tan ({kl})}}}}} & {{Equation}\mspace{14mu} 2} \end{matrix}$

wherein i indicates the imaginary part of a complex number and is defined as the sqrt(−1).

For example, a steel cylinder 3.2 m long and 0.1 m in diameter and a 3800 mm² cross sectional area can be used to represent the combined acoustic impedance of a preload spring and an acoustic tuning element; the acoustic impedance at a first end of the cylinder given a free end at the second end of the cylinder can be calculated using Equation 2. The resulting acoustic impedance contains resonant peaks and nulls which occur at frequencies corresponding to integer multiples of quarter wavelengths of the first resonant frequency. FIG. 9(a) shows the first resonance occurring at a cylinder length of l=λ/4. The resonant impedance peak shown in FIG. 9(a) is too high to be of any use, however the acoustic impedance level on the higher frequency side of the resonance peak is low enough to be useful. FIG. 9(b) shows the magnitude of the cylinder's acoustic impedance across the third acoustic passband of the drill string as shown in FIG. 3. The properties of the tuning element disclosed here is only one possible example; the impedance behavior of the tuning element can be controlled through choice of materials, the length of the tuning element, the mass of the tuning element and the distribution of the mass along the length of the tuning element.

FIG. 6(b) shows a detailed view of the internal components of the acoustic transmitter 601. In particular, a first mechanical interface 613 is shown between the first end coupling 602 and the piezoelectric stack 605, and a second mechanical interface 615 is shown between the piezoelectric stack 605 and both the cylindrical acoustic tuning element 606 and the preload spring 607. At the first mechanical interface 613, in an acoustically matched system the acoustic impedance Z₁ of the first end coupling 602 would be the same as the acoustic impedance of the piezoelectric stack 605. This condition is also true for the acoustic impedance Z₂ at the second mechanical interface 615. However if only the preload spring 607 is applied then the compliance of the preload spring 607 is too high to offer the required acoustic impedance and the output power of the piezoelectric stack 695 is reduced. The addition of the acoustic tuning element 606 reduces the compliance of the preload, restoring the acoustic impedance to the required value. Ideally, the acoustic tuning element 606 has a selected impedance that when combined with the acoustic impedance of the preload spring 607, equals the acoustic impedance at the first mechanical interface 613, i.e. the acoustic impedance of the first end coupling 602.

To demonstrate, given a common piezoelectric material with a density of 7.5 Mg/m³, and a Young's modulus of 9.9*1010 N/m², then a piezoelectric stack with a length of 0.142 m and a cross sectional area of 4200 mm² will have a wave impedance of 114 Kg/s. FIG. 10 shows that the combined acoustic impedance of the tuning element and the preload spring (labeled “cylinder” in FIG. 10) is equal to that of the piezoelectric stack at 640 Hz (“center frequency”), with a useable operating frequency bandwidth across the 600 Hz to 700 Hz bandwidth of the third passband of the drill string. In other words, the usable range of acoustic impedance of the tuning element 606 in this example is between 70 kg/s and 160 kg/s for a selected operating frequency bandwidth of 600-700 Hz. While the usable operating frequency bandwidth of the tuning element in this case is about 15% of the center frequency, the usable operating frequency bandwidth and resulting usable acoustic impedance range of the tuning element can vary based on the physical properties of the piezoelectric stack and enclosure, as well as on the operating conditions. Generally speaking, the acoustic impedance of the tuning element can be within a selected range that maximizes acoustic power transfer from the piezoelectric stack into the enclosure over a selected usable operating frequency bandwidth.

Instead of spacers 409 or a retaining ring 509, other types of adjustable preload means can be used to provide a compressive preload to the transducer via the preload spring. For example, referring to FIG. 7 and according to another embodiment, an acoustic transmitter 701 comprises a threaded nut 709 that is mounted to a mandrel 704 to apply a selected compressive preload to a transducer 705 via a preload spring 707. Like the previous embodiments, an enclosure comprising first and second end couplings 702, 710, an outer housing 703 and the mandrel 704 provides a fluid tight space 708 to house the transducer 705, preload spring 707, and threaded nut 709. Optionally and as shown in FIG. 8, an acoustic tuning element 706 similar to the previous embodiments can be installed to match the acoustic impedance of the transducer 705 with the first end coupling 702, thereby maintaining optimal power output by the acoustic transmitter 701.

While the illustrative embodiments of the present invention are described in detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications within the scope of the appended claims will readily be apparent to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general concept. 

1. A downhole acoustic transmitter for use in downhole communication, comprising: (a) an enclosure comprising a first end coupling, a second end coupling, a tubular outer housing having a first end coupled to the first end coupling and a second end coupled to the second end coupling, and an inner mandrel inside the outer housing and extending between the first and second end couplings such that an annular space is defined between the mandrel and the outer housing; (b) a piezoelectric transducer in the annular space, and having a first end contacting an inner face of the first end coupling in an axial direction; (c) a preload spring in the annular space and having a first end contacting a second end of the piezoelectric transducer in the axial direction; (d) an adjustable preload means contacting the enclosure and a second end of the preload spring such that a compressive force in the axial direction is applied to the preload spring, which in turn compresses the piezoelectric transducer against the inner face of the first coupling.
 2. The downhole acoustic transmitter as claimed in claim 1 wherein the adjustable preload means comprises one or more spacers contacting an inner face of the second end coupling.
 3. The downhole acoustic transmitter as claimed in claim 1 wherein the adjustable preload means is a retaining ring attached to an inner surface of the outer housing.
 4. The downhole acoustic transmitter as claimed in claim 1 wherein the adjustable preload means is a threaded nut attached to the mandrel.
 5. The downhole acoustic transmitter as claimed in claim 1 wherein the piezoelectric transducer comprises an annular stack of annular piezoceramic discs with electrodes between each disc, wherein the annular stack is slidable over the mandrel.
 6. The downhole acoustic transmitter as claimed in claim 1 wherein the preload spring is a metal tube slidable over the mandrel.
 7. The downhole acoustic transmitter as claimed in claim 1 wherein the preload spring comprises one or more metal rods or tubes each extending in the axial direction in the annular space.
 8. The downhole acoustic transmitter as claimed in claim 1 further comprising an acoustic tuning element in the annular space and attached to the second end of the piezoelectric transducer, the acoustic tuning element having a selected acoustic impedance that maximizes power transfer from the piezoelectric stack into the enclosure over a selected operating frequency bandwidth.
 9. The downhole acoustic transmitter as claimed in claim 8 wherein the acoustic tuning element has a center frequency wherein the acoustic impedance of the acoustic tuning element matches the acoustic impedance of the piezoelectric stack and the selected operating frequency bandwidth is up to 15% of the center frequency.
 10. The downhole acoustic transmitter as claimed in claim 9 wherein the acoustic tuning element comprises a metal cylinder having a first end attached to the second end of the piezoelectric transducer and a free second end.
 11. The downhole acoustic transmitter as claimed in claim 9 wherein one or more of a mass density, mass distribution, length and cross sectional area of the acoustic tuning element is selected to provide the selected acoustic impedance.
 12. The downhole acoustic transmitter as claimed in claim 1, mounted in a telemetry tool or a repeater of a drill string, wherein the downhole acoustic transmitter has a configuration selected from a group consisting of: collar-based, clamp-on, and probe-based.
 13. A downhole acoustic telemetry node comprising: (a) one or more sensors for measuring a local borehole environment and one or more mechanical conditions of a drill string; (b) a processor and memory communicative with the one or more sensors for storing measurements taken by the one or more sensors; and (c) the downhole acoustic transmitter as claimed in claim 1 communicative with the processor and memory for transmitting the measurement
 14. A method for acoustic transmission from a downhole location, comprising: (a) applying a compressive preload in an axial direction against a preload spring, which in turn compresses a piezoelectric transducer against an inner face of a first end coupling of an enclosure of a downhole acoustic transmitter, wherein the compressive preload is selected to place the piezoelectric transducer in compression over a range of expected operating conditions of the downhole acoustic transmitter; and (b) applying a voltage to the piezoelectric transducer to generate an acoustic transmission.
 15. The method as claimed in claimed in claim 14 further comprising tuning the acoustic impedance of the piezoelectric transducer by contacting an end of the piezoelectric transducer with an acoustic tuning element having a selected acoustic impedance such that when combined with an acoustic impedance of the preload spring, equals the acoustic impedance of the inner face of the first end coupling, wherein the end of the piezoelectric transducer contacting the acoustic tuning element also contacts the preload spring. 